docs: def _setoct(self, octstring):
          """Reset the bitstring to have the value given in octstring."""
          octstring = tidy_input_string(octstring)
          # remove any 0o if present
          octstring = octstring.replace('0o', '')
          binlist = []
          for i in octstring:
              try:
                  if not 0 <= int(i) < 8:
                      raise ValueError
                  binlist.append(OCT_TO_BITS[int(i)])
              except ValueError:
                  raise CreationError("Invalid symbol '{0}' in oct initialiser.", i)
          self._setbin_unsafe(''.join(binlist))